This invention relates to adhesive compositions and methods, particularly to adhesive compositions for bonding rubber to fabric, reinforcing rubber compounds with fabric, and compositions of rubber bonded to fabric with adhesive compositions.
Reinforced rubber goods are used in a wide variety of consumer and industrial applications. The performance of reinforced molded rubber goods depends on the adhesion of the reinforcement to the rubber. Fabrics made with synthetic yarns tend to be difficult to bond to rubber. In practice several things are done to improve adhesion. As the fibers are drawn a spin finish is applied which may contain an adhesion activator such as an epoxy resin. The yarn may be heat set before or after weaving to relax the orientation put into the fiber by drawing and avoid shrinkage during the rubber bonding curing step. Shrinkage during this step will adversely effect adhesion. Finally, to get good bonding to rubber, an adhesive dip coat is applied to the fabric.
A typical dip coat process to apply adhesive to fabric may involve many steps and is tailored to each application, i.e., a different adhesive may be used depending on the fabric type and the rubber it is to be molded with. Some of the steps may include a pretreatment dip of the fabric with a polyisocyanate dissolved in a solvent. The fabric is then dried and put through a forced air oven to cure the coating and heat set the fabric. This operation may be followed by a subsequent dip treatment in a resin-reinforced latex (RFL) bath, and a second trip through the oven. The composition of these baths may include the addition of other adhesive promoters such as epoxies, phenolics, or the like. An RFL bath is made by preparing a 6% resin solution of resorcinol and formaldehyde or other methylene donor in water. This solution is allowed to react for a certain time at a specified pH and temperature and is then poured into a rubber latex, and the blend is again allowed a certain time to “mature” before using. U.S. Pat. No. 3,307,966, assigned to E.I. DuPont describes such a process.
In practice these types of RFL dip adhesives have several drawbacks, including the following. The shelf life of the adhesive composition depends on several factors including pH, temperature and the exact ingredients and is difficult to control. Typical shelf life can vary from one day to one month after which the material must be disposed of. It is also difficult to test whether or not the RFL adhesive has drifted out of its useful property range. Batch to batch variation is also a problem. Environmental and safety issues are a concern. The adhesives are dried and cured at high temperature (up to 200° C.). In addition to the high energy usage there are emissions of solvents, formaldehyde and other materials. Finally, performance of the RFL adhesive in the end use application is often inadequate. The goal is to have cohesive failure of the rubber matrix rather than adhesive failure at the rubber-fabric interface with the RFL adhesives; this is often only achievable by addition of other additives to the rubber.